RNA interference (RNAi) is a powerful tool for gene function studies and drug development. Currently, application of this technology is largely limited by the efficiency of delivering RNAi-inducing reagents into mammalian cells. The aim of this proposal is to establish techniques to efficiently transfect siRNA-expressing cassettes. We have already developed a system to induce R NAi effects in mammalian I cells using novel DNA-based expression cassettes that offer gr eat advantages over other RNA or plasmid-based methods. We plan to take advantage of the unique design of our existing core technology by using transducing peptides as dell very vehicles. This will be particularly useful for RNAi-based gene silencing in traditionally hard to transfect cells such as primary cultures. T hese techniques can be further integrated into novel platforms for high throughput discovery and adapted for use in live animals. Development of these methods may also lay the foundations for exciting applications of our RNAi technology as therapeutic agents.